1. Field of the Invention
The present invention relates to a gallium-nitride-based semiconductor light emitting device and a fabrication method thereof.
2. Description of the Related Art
A GaN-based semiconductor light emitting device using a gallium-nitride-based compound is a short wavelength semiconductor device used in the wave band of about 300 to 500 nm.
The GaN-based semiconductor light emitting device is fabricated by using the metal-organic chemical vapor deposition method (hereafter referred to as MOCVD method). The MOCVD method is a film forming method of introducing a plurality of source gases into a reactor to make the gases vapor-phase-react on one another at a predetermined temperature and depositing an obtained compound on a substrate. By forming a film while changing species of source gases or component ratios of the source gases, it is possible to obtain a multilayer film made of different compounds or compounds with different component ratios. A GaN-based semiconductor light emitting device is obtained by setting a temperature in the reactor to 900 to 1100xc2x0 C., properly using an organic metal such as trimethyl-gallium (hereafter referred to as TMG) or trimethyl-aluminum (hereafter referred to as TMA), or ammonia (NH3) as a source gas, thereby forming a single crystal layer of a GaN-based compound semiconductor such as (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) on a substrate crystal.
The light emitting characteristic of the obtained device is influenced by the density of line defects, that is, the density of threading dislocations in a crystal. Because a threading dislocation creates a position where no light is emitted, it causes the light emitting characteristic to deteriorate. The threading dislocation appears when a physical misfit between a substrate crystal and a single crystal layer deposited on the substrate crystal is too large. For example, the threading dislocation is caused by the difference in lattice constant or crystal structure or the difference in thermal expansion coefficient. Therefore, it is preferable that a substrate material is the same as that of a single crystal layer to be formed on the substrate. In the case of a GaN-based semiconductor light emitting device, it is preferable to use a GaN single crystal for a substrate. However, it is very difficult to obtain a large GaN single crystal usable as a substrate. Therefore, for example, use of a semiconductor single crystal such as gallium arsenide (GaAs) as a substrate is also considered. However, because the temperature in an MOCVD reactor when forming a GaN single crystal multilayer film to be formed on the substrate ranges between 900 and 1100xc2x0 C., the above semiconductor single crystal is physically unstable in this temperature range. Therefore, sapphire which is physically stable even at a high temperature has been used so far as a substrate, but the lattice constant of sapphire and that of GaN are different from each other by about 14%.
A two-stage film forming method is a method for forming a multilayer film so as to moderate the misfit of the lattice constant. The two-stage film forming method is a method of forming a buffer layer on a substrate at a temperature lower than a conventional temperature and forming a single crystal multilayer film on the buffer layer similarly to a conventional method.
As shown in FIG. 1, a low-temperature buffer layer 102 made of aluminum nitride (AlN) or GaN is formed on a sapphire substrate 101. The low-temperature buffer layer 102 is formed by setting the temperature in an MOCVD reactor at 400 to 600xc2x0 C. After the low-temperature buffer layer 102 is formed, the temperature in the MOCVD reactor is raised to 900 to 1100xc2x0 C. and a semiconductor single crystal layer represented by (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) is successively formed. A GaN underlying layer 103, n-type semiconductor layer 104, n-type cladding layer 105, active layer 106, p-type cladding layer 107, and p-type contact layer 108 are formed on the low-temperature buffer layer 102 in the stated order. A semiconductor light emitting device is fabricated by forming a p-side electrode 109 on the p-type contact layer 108 and an n-side electrode 110 on the n-type semiconductor layer 104.
The initial GaN crystal nuclei of the GaN underlying layer 103 deposited on the low-temperature buffer layer 102 grow on a plane (1 xe2x88x921 0 1) and a plurality of high-order planes nearby the plane (1 xe2x88x921 0 1). Therefore, the nuclei three-dimensionally grow like islands in directions parallel with and vertical to a plane of a substrate. When three-dimensional crystal growth progresses, islands collide with each other and are combined into one body and a high-order growth plane changes into the plane (1 xe2x88x921 0 1) and another stable plane (0 0 0 1). Because Ga atoms reaching the plane (0 0 0 1) migrate on the surface of the plane (0 0 0 1) and are captured into the plane (1 xe2x88x921 0 1), the growth rate of the plane (0 0 0 1) is very small compared to that of the plane (1 xe2x88x921 0 1) and only the plane (0 0 0 1) remains as a growth plane. Thus, the growth plane two-dimensionally grows only in the direction that is vertical to the substrate plane and a film is flattened.
In the process of change from three-dimensional growth to two-dimensional growth, when the pressure in a reactor under film formation is low, the density of crystal nuclei produced is high because of the high diffusion rate of the precursor material and change to two-dimensional growth quickly occurs and thereby, a film is flattened. In this case, because a line defect density increases proportionally to the density of crystal nuclei, the obtained GaN underlying layer 103 has a high threading dislocation density.
However, when the reactor has a high pressure almost equal to atmospheric pressure, the density of crystal nuclei produced is low because of the low diffusion rate of the precursor material and the initial crystal nuclei grows to a large island after competition among islands due to selective growth and changes to two-dimensional growth. Therefore, the threading dislocation density of the obtained GaN underlying layer 103 decreases.
As shown in FIGS. 2 and 3, when a crystal nucleus grows to a large island and then changes to two-dimensional growth, a large pit 103a constituted of the plane (1 xe2x88x921 0 1) easily occurs at a combined portion because islands are slowly combined with each other. Because supply of the precursor material into the deeper part of the pit 103a is reduced, it becomes more bulky as a film further grows and interrupts the smoothness of the film. In this case, even if the n-type semiconductor layer 104 is formed on the pit 103a, the flatness of the layer 104 is interrupted because supply of the precursor material into the deeper part of the pit 103a is reduced and thereby, a pit 104a is produced (refer to FIG. 3). This makes the light emitting characteristic of a device deteriorate similarly to the case of a threading dislocation.
It is an object of the present invention to provide a GaN-based semiconductor light emitting device having a superior light emitting characteristic by reducing the occurrence of a threading dislocation and forming the flat film, and a GaN-based semiconductor light emitting device fabrication method.
A gallium-nitride-based semiconductor light emitting device fabrication method of the present invention is a method for producing the device by forming a film of a nitride semiconductor (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) on a substrate with low-temperature buffer layer by means of accordance with the metal-organic chemical vapor deposition (MOCVD) method, which comprises the underlying layer forming steps of forming an undoped gallium-nitride underlying layer on the low-temperature buffer layer while keeping the pressure in the reactor almost equal to the atmospheric pressure and the flattening step of forming a flattening layer for flattening the surface of the underlying layer.
Moreover, a gallium-nitride-based semiconductor light emitting device of the present invention comprises a structure obtained by forming a nitride semiconductor (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) on a substrate with a low-temperature buffer layer, wherein a underlying layer and a flattening layer containing a Group IV element having a concentration high enough to flatten the surface of the underlying layer are formed on the low-temperature buffer layer.
The threading dislocation density is decreased by forming a GaN underlying layer to be formed on a low-temperature buffer layer while keeping the reactor pressure almost equal to the atmospheric pressure and the number of large pits to be produced is reduced by optimizing a thickness of the GaN underlying layer. Moreover, when an n-type semiconductor layer to be formed on the GaN underlying layer is formed, migration of Ga atoms reaching the plane (0 0 0 1) of the GaN underlying layer is accelerated by containing a group IV element in a source-gas atmosphere, two-dimensional growth is promoted, and pits formed on the GaN underlying layer are smoothed.